Although the electrodeposition of silver from a variety of plating baths has been disclosed in the art, there is still a need for baths capable of functioning effectively for high speed bright silver plating.
As is well known in the art, silver cyanide plating baths have been employed for many years. Many efforts also have been made to develop noncyanide or low cyanide silver plating baths. See, for example, U.S. Pat. Nos. 4,155,817 (Fletcher) and 4,024,031 (Lerner); where the latter patent is directed to an electroplating bath with a low silver content, which is essentially at a neutral pH and operates substantially without free cyanides.
The patent to Fletcher (U.S. Pat. No. 4,155,817) also contains a detailed description of silver plating. When this is considered, along with the disclosures in the prior art cited in this patent as well as in the Lerner patent, one obtains an excellent background in the historical silver plating art, as well as the numerous endeavors to develop electroplating baths having improved characteristics, including high speed deposition of high quality, bright silver deposits.
The patent to Fletcher discloses, inter alia, the use of a divalent (-2) selenium complex together with an electrolyte bath having a free cyanide content of less than 1.5 g/l. The use of alkali metal selenium compounds as brighteners is also disclosed in U.S. Pat. No. 2,613,179 (Wolfson) and 4,121,982 (Fletcher). Wolfson discloses, inter alia, that the use of a selenite of an alkali metal together with alkali metal cyanides and nitrates results in the formation of a high speed bright silver deposit because of the presence of nitrate (100-150 g/l) and potassium selenite up to 1.0 g/l. Wolfson states that the presence of the nitrates permits him to plate up to 100 ASF (10 ASD) and that without the nitrates this current density could not be achieved.
Another relevant patent for the present purposes is U.S. Pat. No. 2,735,808 (Greenspan) which teaches that the use of a glycerol complex and potassium cyanide are necessary to obtain bright silver deposits from an electroplating which is tartarate-free. Greenspan states that his baths must be tartarate-free when using glycerol complex of antimony.
Furthermore, in U.S. Pat. No. 2,777,810 (Ostrow) there is a disclosure that divalent (-2) selenium compound in the presence of antimony compounds and free cyanide gives bright deposits up to 100 ASF.
In the practice of the inventions of these patents, and particularly those in which divalent or hexavalent selenium compounds are used, either by themselves or in combination with antimony, it has not been possible to obtain bright silver deposits at high current densities. Specifically, the current densities achieved in the processes of these patents, current density being a measure of plating speed, have not been significantly in excess of about 100 ASF.
Other silver electroplating baths which have been developed are described by Weiner, Alien Property Custodian Ser. No. 351,241; Egeberg et al, U.S. Pat. Nos. 2,176,668; Nobel et al, 2,783,194; Hofmann et al, 3,186,926; and Todt et al, 3,580,821. In each of these, the silver electroplating baths used contain free-cyanide and selenium compounds, including tetravalent seleneium compounds. In the operation of these baths, however, it has not been found to be possible to achieve bright silver electrodeposits at current densities which are significantly in excess of about 100 ASF.
In a more recent development, as described in Rosegren et al, U.S. Pat. No. 4,265,715, significant increases in the operating current densities of a bright silver electroplating bath are obtained using a cyanide-free bath which contains an organic phosphonate compound. With this electroplating bath, plating speeds (current densities) of up to about 2,000 ASF are achieved. While this is a significant improvement over the prior bright silver plating processes, recent advances in this art now require processes which can operate at even higher current densities and still produce a bright silver electrodeposit. Additionally, the use of organic additives in the Rosegren et al process may require more frequent monitoring and/or control of the bath operation than is generally necessary with plating baths based on only metallic additives.
It is, therefore, an object of the present invention to provide an improved electroplating bath and process for the high speed electrodeposition of bright silver deposits.
A further object of the present invention is to provide an electroplating bath and process for the high speed electrodeposition of bright silver deposits which do not utilize organic phosphonate additives in the plating bath.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.